Coherent detection is used in optical fibre gyroscopes to measure signal phase shift, in long haul optical fibre communication to increase detector sensitivity and in wavelength multiplexed schemes to provide wavelength selective detection. In each of these applications the polarisation state of the signal must be matched to that of the reference &lt;or local oscillator) at the photodetector. However, in conventional monomode optical fibres the polarisation state of the signal can change continuously--in order to realign signal polarisation state one needs polarisation controllers which can be expensive, bulky and unreliable.
Optical fibre multiports can be used to detect the complete envelope of signal phase or in long haul communications to provide "phase diversity"--coherent detection of the signal which is independent of fluctuation is signal phase. Such multiports are difficult to make because they require several fibres to be fused with precise symmetry. A three fibre device has been described by Davis A. W. and Wright S. ("A phase sensitive homodyne optical receiver" IEE Colloquium Digest No 185/30 Paper 11), but while the properties required for a four-fibre device have been Know for some time it is only recently that such a device has been successfully made. Even now the necessary control of the extra fourth mode in the four fibre device is difficult.
It is possible to imagine that a "full diversity", i.e. coherent detection of a signal independent of either its phase or its polarisation state might be provided by arrays of multiports. For example, a polarisation splitter could be used to separate two orthogonally polarised components of a signal into a pair of phase diversity multiports. Alternatively such an array might be used for complete detection of each polarisation state of the signal phase and/or polarisation state. However, such arrays of couplers are bulky and expensive.